Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow

Results are presented from a series of settling column experiments investigating temporal variations in the flocculation characteristics of purely cohesive (kaolin clay) sediment suspensions and cohesive (kaolin) and non-cohesive (fine sand) sediment fraction mixtures. Experimental runs were conducted under controlled hydrodynamic conditions generated by a rigid array of in-phase oscillating grids. The results indicated that rapid initial floc aggregation occurred under low turbulent shear rates, with peak maximal and root-mean-square (r.m.s.) floc sizes (∼ 400 μm and ∼ 200 μm, respectively) attained after relatively short time periods, before reducing with time. By contrast, lower aggregation rates and smaller floc sizes were observed under higher shear conditions, with flocs retaining suspended in the settling column for longer time scales due to the increased turbulence. The mud input concentration displayed some correlation with maximal and r.m.s. floc sizes at higher shear rates but no correlation was apparent at low shear rates. This observed floc behaviour may be attributed to the differences in concentration gradients at high and low shear rates that affect both floc settling rate and time required for flocs to attain equilibrium size. The addition of the fine sand fraction to the kaolin clay suspension reduced both the initial floc formation (i.e. aggregation) rate and the maximal and r.m.s. floc sizes attained throughout the experiments. The reduction in maximal floc sizes appeared to be enhanced by an increase in the ratio of fine sand to kaolin clay content within the mixture.     

The influence of particle size of the dispersed mineral fraction on the settlement of marine and estuarine muds

Considering the sedimentation behaviour of suspensions, four states of different sediment-water mixtures are usually distinguished, these being (in increasing concentration) dilute suspensions, concentrated suspensions, fluid muds and mud deposits. As the concentration values delimiting these states vary considerably from one cohesive sediment to another, a procedure for their quantitative determination is proposed here. This paper deals mainly with the effects of hindered settling in interacting concentrated suspensions and of sedimentation in fluid muds. In contrast to the settling velocity of individual particles in a dilute suspension, which must be studied statistically as a stochastic variable denoted W, hindered settling and sedimentation velocities can be described by a scalar denoted V, as solids at any particular level of concentrated suspensions and fluid muds settle at the same velocity. On the basis of settlement tests carried out in this study and published data on organic-rich cohesive sediments, a concentration-dependent empirical law for a permeability coefficient (k) has been generated for cohesive sediments, using data from ten estuarine and nine marine environments, based on the one-dimensional Kynch theory of sedimentation. Based on the median diameters of the dispersed mineral fraction, the main provenances of the sediments are: fine to very fine clay from tropical marine/estuarine environments, medium clay to very fine silt from estuaries in western France, and fine to coarse silt from marine (harbour) environments in the Normandy region of France. A general trend for the influence of the grain size of the mineral fraction on the permeability coefficient has been established. It is demonstrated that the concentrations delimiting the different states of sediment-water mixtures can also be related to the grain size of the mineral fraction. Thus, hindered settling and sedimentation processes of muds, similar to the marine and estuarine cohesive sediments considered in this paper, can be studied as generic problems parameterized through a defined median diameter of the dispersed mineral fraction. Results for Loire estuary sediments are presented separately, based on specific tests to analyse the influence of experimental conditions on settlement. Moreover, the concentration values delimiting the different sediment-water mixture states have been largely established for this estuary.     

Observations of Floc Sizes in a Muddy Estuary

Measurements are presented of median floc diameters and associated environmental data over spring-tide tidal cycles at two stations in the muddy Tamar Estuary, UK, for winter, spring and summer conditions. The particulate organic carbon and particulate total carbon contents of mudflats and SPM (suspended particulate matter) at the stations, together with other evidence, indicates that much of the SPM was derived from mud sources that were located between the two stations during winter and spring, and from very mobile sediment sources in the upper estuary during summer. Observed in-situ median floc sizes varied widely, from <50 to >500 μm and rapid settling of particles close to HW and LW (high and low water) left only the smaller flocs in suspension. Time-series of depth-averaged median floc sizes generally were most closely, positively, correlated with depth-averaged SPM concentrations. Floc diameters tended to reach maximum median sizes near the time when SPM concentrations were highest. These high concentrations were in turn largely generated by resuspension of sediment during the fastest current speeds. Although such correlations may have arisen because of SPM-driven floc growth - despite fast tidal currents - there is also the possibility that tough aggregates were eroded from the intertidal mudflats and mudbanks. Although a hypothesis, such large aggregates of fine sediment may have resulted from the binding together of very fine bed particles by sticky extracellular polymeric substances (EPS) coatings, produced by benthic diatoms and by other biologically-mediated activity. A rapid reduction of SPM occurred at the up-estuary station within 2.5 h of HW on the flood, when decelerating currents were still relatively fast. It appears that at least two processes were at work: localised settling of the largest flocs and up-estuary transport in which large flocs were transported further into the estuary before settling into the Tamar's ETM (estuarine turbidity maximum) over the HW-slack period. Up-estuary advection of large flocs and their eventual settling would place the down-estuary edge of the ETM above the upper-estuary station during summer, spring-tide conditions. This position of the ETM was observed close to HW during longitudinal surveys of the estuary.     

Hydro-meteorological influences and multimodal suspended particle size distributions in the Belgian nearshore area (southern North Sea)

Suspended particulate matter (SPM) concentration and particle size distribution (PSD) were assessed in a coastal turbidity maximum area (southern North Sea) during a composite period of 37 days in January–April 2008. PSDs were measured with a LISST 100X and classified using entropy analysis in terms of subtidal alongshore flow. The PSDs during tide-dominated conditions showed distinct multimodal behaviour due to flocculation, revealing that the building blocks of flocs consist of primary particles (<3 μm) and flocculi (15 μm). Flocculi comprise clusters of clay minerals, whereas primary particles have various compositions (calcite, clays). The PSDs during storms with a NE-directed alongshore subtidal current (NE storms) are typically unimodal and characterised by mainly granular material (silt, sand) resuspended from the seabed. During storms with a SW-directed alongshore subtidal current (SW storms), by contrast, mainly flocculated material can be identified in the PSDs. The findings emphasise the importance of wind-induced advection, alongshore subtidal flow and highly concentrated mud suspensions (HCMSs) as regulating mechanisms of SPM concentration, as well as other SPM characteristics (cohesiveness or composition of mixed sediment particles) and size distribution in a high-turbidity area. The direction of subtidal alongshore flow during SW storm events results in an increase in cohesive SPM concentration, HCMS formation, and the armouring of sand; by contrast, there is a decrease in cohesive SPM concentration, no HCMS formation, and an increase in sand and silt in suspension during NE storms.     

Laboratory investigation on measuring suspended sediment by portable laser diffractometer (LISST) focusing on particle shape

This paper deals with a laboratory investigation on measuring suspended sediment volume and mass concentration (SSC) and particle size distribution (PSD) by a portable submersible laser diffractometer (LISST-100X) focusing on effects of particle shape and inversion modes on the results. Experiments were carried out in a mixing tank using suspensions of glass beads, fine quartz sand, feldspar and mica powder particles of spherical, rounded, angular and flaky shapes respectively, at various SSCs. SSCs and PSDs measured by LISST were compared to SSCs from gravimetric analysis and to PSDs obtained from image analysis of dry particles and from a non-portable laser diffractometer. Experiments using spherical and rounded particles showed that LISST with the corresponding inversion modes, i.e. spherical and ‘random shaped’, provides PSD and SSC values similar to reference methods. For angular and flaky particles, however, SSCs were found to be overestimated by factors of 1.5 and 8 respectively. Measurements in a mixture of 70% feldspar and 30% mica powders showed that the SSC overestimation factor for mixed particle-type suspensions can be predicted as the weighted sum of the SSC factors of the components and their mixing ratio. For known highly non-spherical particle types, LISST SSCs can be corrected by a gravimetrically determined or predicted overestimation factor. Moreover, correction of overestimated contributions of lower size bins, the range of measureable SSC and time averaging of LISST measurements are addressed. Further investigations are necessary to assess effects of non-spherical particles and mixtures of various particle types on LISST SSC and PSD estimates.     

Particle size of suspended matter in estuaries

Suspended matter usually flocculates into fragile flocs that break up during sampling and analysis. Coulter counter and pipette size analysis are therefore an indication of floc strength. Grain-size distributions (after removal of organic matter) give an indication of the relative amount of grains transported as flocs (as opposed to being transported as single particles).In situ size distributions of suspended matter (flocs) in the Scheldt and Rhine estuaries and the Gironde, as well as results from the Zaire River estuary, indicated that salt flocculation does not appear to be of importance for the formation ofin situ flocs. Also there is no clear relation with particle concentration. In estuaries probably no equilibrium between floc size and concentration is reached because of the rapid variations in turbulence and bottom shear. In the Zaire estuary a decrease in turbulence in the surface water results in flocs of the same size as differential settling over a depth of more than 100 meters.     

The small volume particle microsampler (SVPM): a new approach to particle size distribution and composition

The characterization of trophically and geochemically important suspended particulate matter (SPM) has traditionally relied on bottle sampling and subsequent analysis with Coulter Multisizers and other instruments, which are not sufficient in preserving the in situ size, shape and composition of aggregated particles. The small volume particle microsampler (SVPM) is a sampling device that captures individual particles on filters with minimal disturbance for microscope image analysis of size distribution and composition. Sand grains, microalga (Dunaliella tertiolecta) and laboratory cultivated flocs were used to test the SVPM's ability to determine particle size. For statistical analysis of the SVPM's capabilities, sand grain and algal size distribution, calculated as equivalent spherical diameter (ESD), were compared to Multisizer data while video images provided a comparison for the flocs. Non-aggregated sand particles sampled by the SVPM showed a size distribution that was similar to that of the Multisizer. Aggregated D. tertiolecta flocs were broken up by the Multisizer, and SVPM data indicated a significantly greater mean ESD. The SVPM showed significantly smaller mean ESDs than the video images because of the higher resolution of the sampler for small particles. In terms of particle concentration, the microsampler measured values similar to those of the Multisizer and video camera. The most important feature of the SVPM is its ability to capture aggregates for the analysis of composition, by histological stains or other means. The SVPM is an alternative method of sampling that is more effective in preserving aggregates for laboratory analyses and is less complicated and expensive than in situ optical sampling techniques, especially in documenting the lower end of the particle size spectrum.     

A hindered settling model for the prediction of settling and consolidation of cohesive sediment

The prediction of the settling and consolidation behavior of mixtures of cohesive and non-cohesive sediment is possible using a numerical model that solves the solids mass balance for each fraction. Consistency with the hindered settling theory as well as classical soil mechanics is preserved. Semi-empirical relationships for settling rate and effective stress as a function of density are required as constitutive equations. A simple procedure, using settling column experiment data, for calibration is proposed. The possibility of distinguishing different kinds of particles allows prediction of layered deposits.     

Rheological boundaries of mud: Where are the limits?

“Mud” includes a rheological criterion that implies it exhibits a particular consistency state, the lower boundary of which can be specified through the use of the Atterberg plastic limit, but the upper boundary remains undefined. A relationship between the water content of a hindered settling suspension when its structure changes from fluid-supported to body-supported and its Atterberg liquid limit appears capable of predicting the physical conditions at the moment when the suspension acquires a rheological character sufficient to define the upper surface of mud. This relationship appears consistent and predictable within various marine and estuarine environments.     

颗粒形状对珊瑚砂和石英砂沉降影响的试验研究

在研究海岸侵蚀的过程中,泥沙沉速是一个重要参数,对于侵淤量的计算非常重要。近些年来,由于工程活动、海洋动力和气候等因素变化的影响,珊瑚砂海岸的侵蚀冲刷问题已不容忽视。考虑到珊瑚砂与石英砂在形状、比重上的不同会对其沉速造成影响,套用现有石英砂的沉速公式进行计算并不合适。本文针对珊瑚砂和石英砂进行沉降试验,分析形状对珊瑚砂沉速的影响。结果显示,在小粒径(d 0.5 mm)下形状对珊瑚砂沉速的影响并不明显,在大粒径(d 0.5 mm)下明显减缓了珊瑚砂的沉降速度,套用石英砂的沉降公式计算出的珊瑚砂沉速明显偏大。故通过提出动力形状因子这一参数来表征形状对沉降的影响,推导出考虑了形状影响的沉降公式,能够较为准确地计算砂粒的沉速范围,计算精度随着黏度的增大而提升。     

Measurement of the influence of salinity on floc density and strength

The effective density and the strength of flocs formed in the laboratory from mud from the Tamar Estuary reached a maximum value at a salinity of 10–15‰ within the concentration range studied (0.1–1.0 g liter⁻¹). For a constant salinity and concentration, the density decreases with increasing floc size. The strength of the flocs increases with the floc diameter. However, the strength of the individual particle bonds within the floc decreases with size. Large flocs were relatively more brittle than smaller ones. The results suggest that larger flocs may be disrupted by the formation of unequal fragments.     

Sand ripples generated by regular oscillatory flow

The prediction of ripple geometry is a necessary precursor to the prediction of sand transport under waves for ripple regime conditions. The paper begins with a comparison of four existing methods for predicting the geometry of sand ripples generated by oscillatory flow. The comparison points to substantial differences between ripple dimensions predicted by the methods, especially for field-scale conditions. Ripple geometry experiments carried out in a large oscillatory flow tunnel are then described. The experiments involved a range of sand sizes and sinusoidal and asymmetric flows with periods and velocities typical of field conditions. Comparison of measured and predicted ripple geometries leads to the recommendation that the method of Mogridge, Davies and Willis be used to predict ripple geometry for field-scale oscillatory flows. The Nielsen method yields good predictions of ripple length, but the rapid fall-off in ripple steepness predicted by the Nielsen method at high mobility number is not supported by the measurements. The lengths and heights of symmetric ripples produced by sinusoidal flows are found to be similar to the lengths and heights of asymmetric ripples produced by “equivalent” asymmetric flows. Three-dimensional ripples occur with fine sand in long-period flows typical of field conditions. The dimensions of these ripples cannot be predicted using methods developed for two-dimensional ripples. Previously suggested criteria for predicting the occurrence of three-dimensional ripples fail when tested against a wide range of flow and sand conditions. The occurrence of three-dimensional ripples and the effects of ripple and flow history on ripple geometry require further research.     

Dynamics of suspended particles in coastal waters (southern North Sea) during a spring bloom

Measurements of suspended particle concentration, size and settling velocity were made at a shallow site in the southern North Sea during a spring phytoplankton bloom. The site was characterised by strong differences in surface and near-bed residual flows; therefore particle processes in each layer are effectively decoupled as long as the water column is stratified. Four distinct energetic events during the observation period caused variation in the characteristics and behaviour of the particle population: (1) moderate spring tides with low wave activity; (2) strong winds, increased wave activity; (3) strong spring tides; (4) weak neap tides. During Event 1 weak tidal resuspension occurred, median particle diameter was relatively large, but median settling velocities of both chlorophyll and total SPM were low. During the higher energy Events 2 and 3 there was resuspension of relatively small, high-density particles producing high median total SPM settling velocities but low median particle diameter. In addition, a phytodetrital fluff layer, characterised by high chlorophyll settling velocity, was resuspended and dispersed during storm conditions (Event 2). During calm, weak neap tides (Event 4) there was negligible resuspension and enhanced particle settling and deposition, particularly in the phytodetritral component of the particle population, allowing rapid replenishment of the benthic fluff layer. This work indicates the relatively rapid rate at which fluff layers can be formed and dispersed, and highlights the need for high frequency measurements. The range of contrasting physical conditions over which the data-set was collected makes it an ideal candidate for parameterising and validating suspended sediment dynamics models.     

Predictability of wave-induced net sediment transport using the conventional 1DV RANS diffusion model

Based on a large database of laboratory experiments, the predictability of the conventional one-dimensional vertical Reynolds-averaged Navier–Stokes (RANS) diffusion model is systematically investigated with respect to wave-induced net sediment transport. The predicted net sediment transport rates are compared with the measured data of 176 physical experiments in wave flumes and oscillating water tunnels, covering a wide range of wave conditions (surface, skewed, and asymmetric waves with and without currents), sediment conditions (fine, medium, and coarse sands with median grain diameters ranging from 0.13 to 0.97 mm) and bed forms (flat beds and rippled beds), corresponding to various sediment dynamic regions in the near-shore area. Comparisons show that the majority (73 %) of predictions on a flat bed are within a factor 2 of the measurements. The model behaves much better for medium/coarse sand than for fine sand. The model generally underpredicts the transport rates beneath asymmetric waves and overpredicts the fine sand transport beneath skewed waves. Nevertheless, the model behaves well in reproducing the transport rates under surface waves. A detailed discussion and a quantitative measure of the overall model performance are made. The poor model predictability for fine sand cases is mainly due to the underestimation of unsteady phase-lag effect. It is revealed that the model predictability can be significantly improved by implementing alternative bedload formulas and incorporating more physical processes (mobile-bed roughness, hindered settling, and turbulence damping).     

Internal recycling of particle reactive organic chemicals in the Chesapeake Bay water column

Hydrophobic organic contaminants (HOCs) may be used as tracers of particle dynamics in aquatic systems. Internal cycling of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) were studied in the mesohaline Chesapeake Bay to assess the role of resuspension in maintaining particle and contaminant inventories in the water column, and to compare settling and suspended particle characteristics. Direct measurements of sediment resuspension and settling conducted in conjunction with one of the sediment trap deployments indicate reasonable agreement between measurements of particle flux using the two different methods. Organic carbon and PCB concentrations in settling solids collected in near-surface sediment traps were remarkably lower than concentrations in suspended particles collected by filtration during the trap deployments, but higher PAH concentrations were found in the settling particles. The different behaviors of PAHs and PCBs in the settling particles are due to their different source types and association to different types of particles. Sediment trap collections in near bottom waters were dominated by resuspension. Resuspension fluxes of HOCs measured 2 m above the bay bottom were as high as 2.5 μg/m² day for total PCBs and 15 μg/m² day for fluoranthene, and were 25 and 10 times higher than their settling fluxes from surface waters, respectively. HOC concentrations in the near bottom traps varied much less between trap deployments than HOC concentrations in the surface traps, indicating that the chemical composition of the resuspended particles collected in the near bottom traps was more time-averaged by repeated resuspension than the surface particles.     

Settling of Kaolinite in Different Aqueous Environment

Settling characteristics of soils carry great importance for geotechnical engineers since sediments properties are formed during the settling of soil particles in an aqueous environment. In this study, settling characteristics of kaolinite are investigated. Different ionic strengths of NaCl, CaCl₂ and AlCl₃ were considered as a function of pH in aqueous environment of varying solid concentrations. Factors affecting the settling characteristics and fabric of kaolinitic sediments have been identified. The results of the study reveal that kaolinite settles in either flocculated or dispersed forms depending on pH and ion concentration. Flocculated settling occurs in acidic pH due to formation of flocs in edge-to-face structure with increasing positive charges at the particle edges. Dispersed settling occurs in alkaline pHs when ionic strength is low. When ionic strength is increased in alkaline pHs, kaolinite particles settle in flocculated form. Furthermore, the results show that pH has a significant role on the final sediment thickness or void ratio of kaolinite. Densely packed structures in alkaline and loosely packed structures in acidic aqueous environments are formed depending on pH level. Results also show that as the solid concentration increases, the settling rate decreases due to buoyancy effect. Finally, the zeta potential of kaolinite is correlated with the final sediment thickness or void ratio of kaolinite as a function of pH. This correlation proves that there is a good agreement between zeta potential and the final sediment thickness or void ratio, especially when the soil is settled in a dispersed form.     

Ripples in intertidal mud—a conceptual explanation

On protected mudflats and along sheltered tidal channel margins, wave- and current-generated ripples are frequently observed on surficial and subsurface mud beds, although such bedforms are generally not thought to occur in cohesive sediments. In this paper, examples of such ripple marks in the German Wadden Sea (back-barrier tidal flats of Spiekeroog island) and also along the west coast of Korea (Baeksu tidal flats) are documented and analyzed. The mud ripples are 5–8 cm in spacing and 0.3–0.8 cm in height, and are composed of slightly sandy to virtually pure mud (80–98% mud content). For the Spiekeroog study area, a comparison of in situ particle-size measurements of suspended matter and of dispersed mud collected from the ripples shows that the former consists of low-density flocs which are considerably larger than the constituent grains of the latter. To assess local wave effects, near-bed orbital velocities and orbital diameters were calculated on the basis of standard wave theory using estimated wave parameters at the time of the study (June 2004) as well as wave data recorded nearby within the back-barrier tidal basin. The relationships between grain size, morphometric ripple parameters, and the near-bed orbital diameter show the wave-generated mud ripples to be of the orbital post-vortex type. It is demonstrated that only short-period shoaling (intermediate water depth) waves with periods of 1.5–2.5 s and heights of 0.1–0.5 m are able to generate and maintain such ripples. Corresponding near-bed orbital velocities range from 8–32 cm s^–1 and near-bed orbital diameters from 6.25–10 cm. It can be anticipated that increased current shear and turbulence associated with higher and longer waves prevent ripple formation due to the resuspension of settled mud, and the breakdown of suspended flocs and aggregates into smaller particles which then tend to remain in suspension. The most plausible explanation for the formation of the mud ripples is that mud flocs and aggregates deposited from suspension around high-water slack tide under moderate weather conditions initially respond as single (non-cohesive) particles which are hydraulically equivalent to ambient very fine sands. During exposure at low tide, gradual loss of water transforms the rippled mud into increasingly more cohesive mud drapes which are more resistant to erosion. Unless destroyed during high-energy events, the mud ripples may remain intact long enough to become buried and thereby preserved. Indeed, occasional but persistent observations of ripples in sub-Recent to ancient mudrocks document their preservation potential.     

Shallow‐water sand bars on the Ruamahanga River delta Lake Wairarapa

In the shallow waters of Lake Wairarapa, a multiple series of sublimnic sand bars has developed around the outer edge of the Ruamahanga River delta. Shore‐normal movements in bar positions occur in response to changing lake level and wave conditions. Unlike many bar systems, most of these movements are lakeward of the breaker zone. Sediments on the bar crests are well‐sorted fine sands; the troughs are a mixture of fine sand and mud settling out of the turbid lake waters during calm conditions.     

Over time and space changing characteristics of estuarine suspended particles in the German Weser and Elbe estuaries

Fine cohesive, suspended sediments appear in all estuarine environments in a predominately flocculated state. The transport and deposition of these flocs is influenced by their in-situ and primary particle size distribution. Especially the size of the inorganic particles influences the density and hence the settling velocity of the flocculated material. To describe both the changes in primary particle size of suspended particulate matter as well as the variability of floc sizes over time and space, the data of In-Situ Particle-Size Distributions (ISPSDs), Primary Particle Size Distributions (PPSDs) and Suspended Sediment Concentrations (SSCs) were collected. For this, Laser In-Situ Scattering and Transmissiometry (LISST) measurements as well as the water samples were collected in the German Elbe and Weser estuaries, covering seasonal variability of the SSC.The data of the ISPSDs show that the inorganic and organic Suspended Particulate Matter (SPM), as found in the Elbe and Weser estuaries, mostly appears in a flocculated state. The substrate for organic matter is mainly imported from the seaside and transported into the estuaries as indicated by an upstream decrease of the amount of fine particles. In winter, when the freshwater discharge is high, different PPSDs are found in the case of the Elbe estuary in the Turbidity Maximum Zone (TMZ) as well as in the landward and in the seaward sections close to the TMZ. In summer, the distance between the seaward and the landward section is too low to obtain an individual PPSD within the Elbe TMZ.A missing correlation between the PPSD and ISPSD shows that the inorganic constituents do not have an influence on the in-situ floc size. Although flocs aggregate and disaggregate over a tidal cycle and with changing SSC, they do not change their PPSD. The microflocs are therefore strong enough to withstand further breakage into their inorganic constituents.